Feral pigs cause environmental, biodiversity, and agricultural damage and pose a risk for the spread of diseases that can be on a large geographical scale so feral pigs have direct impact on the environment, agricultural production, rural industries and semi urban amenity.
In Australia, feral pigs are estimated to number in excess of 4 million with some estimates as high as 25 million. Feral pigs occupy some 40% of the land mass of Australia and can breed rapidly in favourable conditions. These population estimates mean that at times of peak abundance there may be more feral pigs in Australia than grazed cattle. Feral pigs inhabit, and are well adapted to a wide range of habitats that include sub-alpine, temperate, sub-tropical, tropical and arid zones, and they are present in most Australian states and territories.
Furthermore, in the United States, the presence of some 8 million feral pigs (also referred to as hogs, boar, or swine) has been reported in some 34 states ranging from California to Virginia, the majority residing in Texas and other southern states including Louisiana, Mississippi, and Florida. Feral pigs are the most abundant introduced ungulate in the United States and their density within ranges and range areas are expanding rapidly.
Feral pigs have a very high fecundity and frequently raise litters in excess of 6 piglets per breeding sow. Thus, the capacity for feral pig populations to respond to control measures or to totally exploit a food supply is great, so large-scale and sustained control measures to lower populations are needed.
Feral pigs adversely impact agricultural production, environments and ecosystems. A number of studies have identified a range of environmental and agricultural problems arising from feral pig infestations (Alexiou (1984) Effects of feral pigs (Sus scrofa) on sub-alpine vegetation at Smokers Gap, ACT, Proceedings of Ecological Society of Australia, 12: 135-142; Tisdell, C. A., (1982) Wild Pigs: Environmental Pest or Economic Resource?Pergamon Press, Sydney; Miller, B. and Mullette, K., (1985) Rehabilitation of an endangered Australian bird: the Lord Howe Island Woodhen, Tricholimnas sybvestris, Biological Conservation, 34: 55-95; Mitchell, J. and Mayer, R., (1997). Digging by feral pigs in the wet tropics world heritage area of north Queensland, Wildlife Research, 24: 591-601; Choquenot, D., McIlroy, J. and Korn, T., (1996) Managing Vertebrate Pests: Feral Pigs (Ed. M. Bomford) Bureau of Resource Sciences, Australian Government Publishing Service, Canberra 163 pp; Mitchell, J., (2000) Ecology and management of feral pigs in tropical rainforest, Unpublished PhD Thesis, James Cook University of North Queensland, Townsville; Hone, J., (2002) Feral pigs in Namadgi National Park: dynamics, impacts and management, Biological Conservation 105: 231-242); Singer, F. J., Swank, W. T., and Clebsch, E. E. C., Effects of wild pig rooting in a deciduous forest., Wildlife Management 48: 464-473; Lacki, M. J., and Lancin, R. A., (1986) Effects of wild pigs on beech growth in Great Smoky Mountains National Park, Journal of Wildlife Management 50: 655-659. The key points from these various studies are summarised below.
The predatory behaviour of feral pigs causes major economic damage for animal production enterprises over wide areas. The damage is so severe that some areas cannot sustain productive grazing of livestock such as sheep due solely to the widespread infestation of feral pigs. The species' impact on agricultural production in Australia has been conservatively estimated to be in excess of 100 million Australian dollars annually.
Feral pigs also cause significant damage to the environment due to their destructive foraging habits which include digging for plant roots or soil fauna including worms (rooting) and consumption of plants and plant root systems. This destructive behaviour, and their need to access areas of food resource or water wallowing points, can damage infrastructure including fencing, dams and levy banks and also causes damage wide areas of fragile riparian habitat. Feral pig fecal contamination of waterways and water storages is also a concern. Their effect on native animal species is unknown but is likely to be severe in view of their predatory behaviour and competition for food resources. Feral pigs are known to aggressively predate new born lambs to such an extent that profitable sheep farming has been discontinued in large areas as a result of the incursion of feral pigs into these areas. It is likely also that feral pigs adversely impact on native wild life species though these impacts are not well quantified.
Apart from direct damage to grazing enterprises and the environment, feral pigs also harbour several major human and animal diseases. Many diseases are zoonotic and the pig provides an ideal amplifying host. Japanese encephalitis virus, leptospirosis, brucellosis and melioidosis have already been detected in feral pigs in Australia. However, an even greater risk comes if there is an incursion of foot and mouth disease (FMD) virus into the feral pig population, where the cloven hoofed pigs provide an amplifying host and virus carrier that is widely distributed and highly mobile.
In the United States, pseudorabies virus (PRV) has been eradicated from domestic pigs however PRV continues to circulate in feral pig and raccoon populations. Accordingly, feral omnivore populations can also be a reservoir for fresh PRV outbreaks.
More recent findings have linked the transmission of Ebola viruses in non-human primates to contact with feral pig populations which have been reported to carry the virus in lesions focussed in the lungs without a lethal outcome for the pig.
Details of the environmental, human health, animal health and agricultural production problems that arise already, or which might arise, from an unchecked expansion in feral pig numbers are provided in the book “Managing Vertebrate Pests: Feral Pigs” (Choquenot, D., McIlroy, J. and Korn, T., (1996) Managing Vertebrate Pests: Feral Pigs (Ed. M. Bomford) Bureau of Resource Sciences, Australian Government Publishing Service, Canberra 163 pp. Infestation of other omnivorous species such as raccoons, collared peccaries, opossums, possums and rodents can give rise to similar adverse agricultural, environmental, financial and health concerns in various countries.
There is therefore a considerable effort focussed on a reduction of the risks posed by feral omnivorous species in Australia, United States, New Zealand, Italy and other parts of the world which have unchecked populations of such species.
Despite their impact, the control of omnivores such as feral pigs, is generally time-consuming, ad-hoc and reactive rather than pro-active management. Many techniques are currently employed for mainly localized control. The methods include baiting with poison baits, shooting (with ground teams or by helicopter based marksmen), trapping (for destruction or harvesting), and fencing (to attempt to exclude pigs from an area). It is recognized that broad-scale and integrated baiting campaigns are most cost-effective for reducing and maintaining feral omnivore populations at low densities across large areas. Typical baiting campaigns include ground baiting after pigs are clustered to a site of habitual feeding or aerial baiting where the bait is dropped from an aircraft into the loci of the target omnivore population to be controlled.
Lethal baiting campaigns include the use of various poisons, for instance, sodium fluoroacetate (1080) which is placed in or deposited on cereal grains, fermented grain, compressed bran/pollard pellet baits, fresh or dried meat, offal, carcasses, lupin seeds, and fruit and vegetables and in manufactured baits. Of these, the use of soaked or dry wheat grain or fresh meat baits are the most common. Feral pigs have also been baited with warfarin soaked into grain or by applying yellow phosphorus suspended in carbon bisulphide onto carcass offal that is scavenged by the pigs. Neither of the last two options are considered to be humane and both may give rise to unintended adverse impacts on non target animals such as birds.
Of the various control means discussed above, poison baiting of feral pigs and other omnivore populations is recognised as one of the most effective means of controlling such populations and reducing the damage they cause. Unfortunately however, two of the main problems with many of the bait types made from grains or meat and carcass offal or pellets is that they require high dose levels of toxicants and that they exhibit poor target specificity. Accordingly, while the commonly employed baiting campaigns may prove effective in controlling feral omnivore (e.g. pig) numbers in a particular area, such campaigns may also adversely affect individuals of other species of animals which may be desired or native species of animals or birds which come into contact with the baits.
Other disadvantages of the present baiting regime can be attributed directly to the specific poison used. For instance, a disadvantage of 1080 is that feral pigs appear to be relatively resistant to the effects of the poison compared to rabbits, foxes, and wild dogs for which it is a more ideal poison. For example, during captive trials with bait delivered 1080, (McIlroy et al, Australian Wildlife Research 16: 195-202) dingos required 0.11 mg/kg to receive an LD50 dose whereas feral pigs were reported to require at least 1 mg/kg and some as high as up to 4.11 mg/kg (O'Brien et al, Australian Wildlife Research 15: 285-291) this being up to a 40 fold multiple of the dose used for canid pests.
Moreover, while the terminal toxic events associated with 1080 toxicosis are not thought to be accompanied by conscious pain, there are disturbances in nervous control of muscle function due, it is thought, to accumulation of citrate in blood and its ability to chelate and remove extracellular calcium ions, thus effects on behaviour that can appear unpleasant to the untrained observer. Humans that have recovered from nearly lethal exposures to 1080 have not recalled pain after the event however the final phases of toxicosis have been likened to hypoglycaemic or epileptic fitting. Sodium fluoracetate is presently one of the best toxin choices for feral pig management in Australia but this chemical is not available for pig control in the USA. Moreover, the high doses of sodium fluoroacetate that are required to get reliable lethal; control of feral pigs mean that this is not a perfectly suitable toxin for pig management especially since such high doses may put potential non-target species at risk.
People poisoned with other toxins such as phosphorus (CSSP) or strychnine have reported substantial pain and suffering and it is highly likely that such poisons are too inhumane to be used to control feral animals such as a pig. Similarly, while warfarin is used therapeutically in low doses for humans suffering from blood clotting disorders and this use is not associated with pain, the use of this anticoagulant in large animals such as feral pigs may give rise to delayed effects, painful haemorrhaging and protracted suffering in some animals and therefore it is also not a preferred poison for this application.
This would indicate that none of these poisons and conventionally used food based baits, are perfectly suitable or represent perfectly humane alternatives for eradicating or controlling this pest species.
Nitrite salts have been recently reported to be potentially suitable for the control of feral pigs (Sus scrofa) and other omnivorous animals such as pest brush tail possums (Trichosurus vulpecula) in New Zealand. Sodium nitrite in particular is commonly approved and used for the preservation of many foods where it reduces the risk of bacterial contamination and improves the colour of certain meat products. While the use at low doses as a food preservative poses low risk, in high doses it causes the conversion of normal haemoglobin to methaemoglobin. Since methaemoglobin is unable to effectively transport oxygen to the brain and other tissues, an animal that achieves high levels of methaemoglobin suffers from an acute metabolic anoxaemia and will become unconscious and die. The effect is similar to the effects of carbon monoxide gas in preventing oxygen transport and is therefore more humane and faster than the actions of most if not all other poisons. Moreover, relative to other animals, the pig (also known as swine or hog) has a deficiency of the enzyme methaemoglobin reductase that is present at higher levels in many other animals and acts to protect animals from the accumulation of excessive levels of methaemoglobin. The pig is thus especially susceptible to this type of poison as it has a deficiency in the means to reverse the poisoning process of methaemoglobin formation.
Despite its effectiveness as a toxicant, sodium nitrite is an unstable molecule that can oxidise and which can react with other chemicals that are present in foods. Sodium nitrite is highly hygroscopic and will degrade in water after dissolution of atmospheric carbon dioxide in the water to form carbonic acid and due to the potential of nitrite to deprotonate the water molecule. The number of potential degradation pathways for interaction with other components of a bait comprising sodium nitrite are large but one breakdown products is nitric oxide gas (NO) that itself is unstable but which can act as a corrosive agent, a chemical messenger and a vasodilator, and which may oxidise to nitrogen dioxide gas (NO2) or may be converted to nitric acid (HNO3) or form nitrous acid (HNO2) in the presence of water or upon contact with mucous membranes such as in the sensitive pig nose. Sodium nitrite is also salty tasting and pigs are generally aversive to high salt diets. Thus the use of sodium nitrite poses formulation challenges to achieve a bait that is stable during storage and use but at the same time is able to deliver larger quantities of the toxicant to the pig in a bait that remains palatable to the pig.
WO 2010/151150 to MacMorran and Eason discloses a nitrite bait formulation in which the nitrite within the bait is encapsulated with zein protein with an approximate amount of nitrite of 80% wt/wt. The results shown in the examples are those obtained with the use of freshly prepared baits. In the hands of the present inventors, these baits, while effective when fresh, degrade over time while stored, decreasing their effectiveness and palatabilty over time. It is postulated that this degradation is taking effect based on the cascade of nitrite breakdown conditions discussed above.
Accordingly an improved method is required to protect the sodium nitrite in the bait carrier system to achieve stability of the formulation over useable shelf-lives by, in part, reducing the production of noxious or aversive breakdown products which is thought to diminish the effectiveness of the bait. If such stability can be achieved this would in turn reduce the possibility of the target pig detecting the nitrite and lead to an increase in voluntary uptake. Any new variation of a bait formulation will still need to also provide effective release of the nitrite into the animal to induce high levels of methaemoglobinaemia and thus cause a quick and painless death without suffering.
The present invention serves to address these long term stability shortcomings which currently exist in the art while at the same time providing an effective bait formulation which serves to humanely control feral omnivorous pest populations.